Lamb poses a greater theoretical risk of spreading BSE to humans than beef, scientists have warned.

As many as 150,000 Britons could die from eating infected meat if sheep as well as cattle have the disease, say experts at Imperial College, London, UK.

The current risk from sheep could be greater than that from cattle

Professor Neil Ferguson, Imperial College, London

But the country's Food Standards Agency (FSA) says the risk of BSE in sheep remains theoretical. The agency is not advising against the consumption of lamb.

The scientists' warning is based on a theoretical prediction of the potential health risk if BSE has passed to sheep and is spreading through the national flock.

Professor Neil Ferguson, leader of the research team, said: "Our latest analysis shows that the current risk from sheep could be greater than that from cattle, due to the more intensive controls in place to protect human health from exposure to infected cattle, as compared with sheep."

Screening call

The team says that under the "worst case scenario" the human death toll from variant Creutzfeldt-Jacob disease (vCJD) could rise from 50,000, if beef was the only source, to 150,000.

But if precautionary measures were taken, the potential health risk from sheep could be reduced by up to 90%, the team adds.

The researchers are calling for urgent screening of more of the 40 million sheep in the UK for signs of BSE. Sheep are known to suffer from a related brain disease, known as scrapie.

There is no evidence that scrapie can pass to people. But there are fears that symptoms of BSE in sheep could be confused with those of scrapie and mask the spread of the disease.

The team recommends that current measures applied to cattle be extended to sheep, including:

Restrictions on the age of sheep slaughtered for food

A ban on offal and spinal cord tissue entering the human food supply.

Certain sheep tissue is already banned from human consumption across the European Union as a precaution.

The FSA's position is that the risk of BSE in sheep remains theoretical and the agency is not advising against the consumption of lamb

Food Standards Agency statement

The spleen is removed from sheep aged six to 12 months.

The skull, brain, eyes, tonsils, spleen, and spinal cord are removed from animals more than a year old.

The Human BSE Foundation, which represents families bereaved by vCJD, said the government should introduce the safety controls suggested by scientists.

Secretary Frances Hall, whose son Peter died from the disease in 1996, said: "If it is in sheep, people could have been eating contaminated meat for years.

"It's very sad to think that more families might be having to go through the same nightmare we've gone through needlessly."

Screening programme

The FSA said the study results would be considered as part of an on-going review of precautionary measures.

It said: "The FSA's position is that the risk of BSE in sheep remains theoretical and the agency is not advising against the consumption of lamb.

Its chairman Sir John Krebs told BBC Radio 4's Today programme that the clean- up of the sheep flock by the Department of Environment and Rural Affairs (Defra), would take several years.

"A key question is do we need to do more to protect public health."

Sir John said an open debate about the evidence was already under way and would continue.

Defra said a new sheep screening programme was launched this month.

But Professor Tim Lang, of Thames Valley University, told Today he was unhappy that Defra was responsible for cleaning up the sheep flock.

"One of the many things this sorry saga has taught us is that we couldn't trust public health controls to be run by a ministry in charge of production."

He said a decision needed to be made whether Defra or the FSA was in charge of BSE policy.

BSE facts

vCJD: variant Creutzfeldt-Jacob disease is an untreatable and invariably fatal disease in humans which is similar to BSE in cattle and scrapie in sheep

Scrapie: A sheep disease with similar symptoms to BSE that has been around for centuries but has never been shown to affect people.

Professor Peter Smith, chairman of the Spongiform Encephalopathy Advisory Committee, which advises the UK Government on BSE and vCJD, said the study highlighted areas where more data was needed.

He said: "It's a theoretical study, and should not necessarily increase people's concern about BSE in sheep."

The Imperial College research is published in the scientific journal Nature.